Construction and cooling arrangement for grooved cathode and associated electrodes



March 10, 1970 J. E. BEGGS CONSTRUCTION AND COOLING ARRANGEMENT FOR GROOVED CATHODE AND ASSOCIATED ELECTRODES 2 Sheets-Sheet 1 Filed Sept.- 28, 1967 w WW 0 2 5 0 H U 5 M W 2 H r I B 8 w d M 6 m 4 2 H 9 w 5 lax H 1 2 //W a 4 y Ab h F 0 1. 8 I 3 1/ 5 u 2 2 6 4.

March 10, 1970 J. E BEGGS CONSTRUCTION AND COOLING ARRANGEMENT FOR GROOVED CATHODE AND ASSOCIATED ELECTRODES 2 Sheets-Sheet 2 Filed Sept. 28, 1967 Fig. 3.

COOL/N6 FLU/0 /n venfor James E 50995 by $1 His Afforney- United States Patent 3,500,107 CONSTRUCTION AND COOLING ARRANGEMENT FOR GROOVED CATHODE AND ASSOCIATED ELECTRODES James E. Beggs, Schenectady, N.Y., assignor to General Electric Company, a corporation of New York Filed Sept. 28, 1967, Ser. No. 671,425 Int. Cl. H01j 1/45, 19/28 US. Cl. 313-348 Claims ABSTRACT OF THE DISCLOSURE An electron discharge device of the planar electrode type has a single supporting insulator upon which a grooved cathode and a control electrode rest with freedom to move to prevent distortion from unequal coefficients of expansion. The control electrode which consists of a large mass ring-like member with large bars that fit into the cathode grooves encircles the cathode and provides an external connection for cooling fine grid wires bonded across the large bars either by direct thermal conduction from the electrode ring to an external circuit, or by passing cooling fluid through interconnected passageways in the ring and bars. The bars and electrode ring are formed of a material having high thermal conductivity.

This invention relates to high-frequency electron discharge devices and in particular to the construction of and cooling system for electrodes in a high-frequency electron discharge device having a grooved cathode and associated electrodes to provide high power output. The invention herein described was made in the course of or under a contract with the United States Army Electronics Command.

In my US. Patent 3,334,263, assigned to the assignee of the present invention, a type of electron discharge device is described and claimed, in which device the cathode has channels or recesses which receive relatively massive bars which form a portion of the grid electrode. The bars, in turn, are joined to a circumferential metal ring which encircles the cathode and a plurality of fine grid conductors are supported on the bars in a transverse direction to that of the bars. While this construction permits operation at high frequencies and relatively high power, there is always the desire to obtain even greater power output at such high frequencies. To accomplish such an objective requires operating at still greater current densities at Whi h the fine wires of the grid tend to distort because of the inability to adequately cool the center portion of such a grid. Such distortion at high temperatures destroys the necessary interelectrode spacing of the device.

Still another problem encountered in operating miniaturized discharge devices at high frequencies and high powers is that, at the temperatures of operation, it is difficult to maintain a stable, rigid structure and in particular to maintain accurate inter-electrode spacing between the cathode and grid. If elforts are made to prevent expansion and contraction of the component elements, the result is either distortion of the electrodes themselves or rupture of the seals between the electrodes and the envelope of the device.

Accordingly, it is an object of my invention to provide a new and improved electrode structure for a miniature type electron discharge device which permits cooling of all portions of the control grid and increased power output.

. It is another object of my invention to provide an electrode structure for a miniature type electron discharge device which maintains a stable, rigid structure at high power outputs.

It is another object of my invention to provide accurice ate reference supporting surfaces for both the cathode and the control grid of a miniature type electron discharge device to maintain accurate interelectrode spacing at high power operation while permitting limited movement of the electrodes at elevated temperatures to avoid their distortion.

It is another object of my invention to provide an electrode structure for a miniature type electron discharge device which permits 'use of external circuits to cool internal electrodes.

It is still another object of my invention to provide an improved electrode construction which eliminates distortion of electrodes and rupture of seals while permitting expansion and contraction of the component elements.

One of the features of my invention is the use in an electron discharge device of the type having a planar anode and an opposed circular cathode with a plurality of spaced parallel grooves, of a control electrode consisting of overlying rows of very fine and thicker high thermal conductivity Wires supported on a relatively massive structure which provides both an external electrical connection and also an arrangement in which heat is conducted from the grid to an external circuit to facilitate cooling even the center portion of the very fine grid wires. In one embodiment a plurality of high thermal conductivity bars extending into the grooves of the cathode is supported by a relatively massive ring which encircles the cathode, the bars and the ring having interconnected passageways so that a cooling fluid can be passed through the passageways to conduct heat from even the innermost portions of the grid structure.

Another feature of my invention consists in supporting both the cathode and the encircling ring of a control electrode from accurately positioned insulating reference surfaces and permitting both elements to expand and contract during operation of the device Without distorting the electrodes or rupturing the seals.

Still another feature is the flexible arrangement by which a high thermal conductivity metal member which provides cooling for the fine wires of a control grid of a miniature type electron discharge device is passed through a pair of adjacent low thermal conductivity high expansion members which form part of the envelope of the device without disturbing the vacuum seal of the device.

The subject matter which I regard as my invention is particularly pointed out and distinctly claimed in the concluding portion of the specification. The invention, however, both as to organization and method of operation, together with further objects and advantages thereof, may best be understood by reference to the following description taken in connection with the accompanying drawings wherein like reference characters refer to like elements and in which FIGURE 1 is an elevation View, partly in section, of a high frequency, high power, electron discharge device according to the present invention.

FIGURE 2 is a perspective view, partly in section, illustrating features of the cathode and control electrode of the discharge device in accordance with the present invention.

FIGURE 3 is an elevation view, partly in section, of a modification of the device of FIGURE 1.

FIGURE 4 is a perspective view, partly cut away, i1- lustratiug features of the device of FIGURE 3.

FIGURE 5 is a plan view, partly in section, of the control electrode structure of the device of FIGURE 3, and

FIGURE 6 is a section along the line 66 .of the control electrode structure of FIGURE 5.

In FIGURES 1 and 2 a high power, high frequency electron discharge device embodying the present invention includes a stud-shaped anode 1 having a circular planar surface, a cathode 2 having an active surface generally conforming to and spaced from that of the anode, the active surface of the cathode having raised planar emitting portions 3 coated with appropriate electron emitting material, and a control electrode 4 interposed between the anode and the cathode emitting surface. The electrodes are quite small in diameter, approximately on the order of one inch or less, in order to reduce inter-electrode capacitance, and thereby facilitate high frequency operation of the device. High power output operation is secured in accordance with the present invention by an unusually high emission current density of the order of two amperes per square centimeter or more. The cathode 2 is heated to a high temperature by a bonded heater element 5 of the type set forth and claimed in the co-pending application of August I. Kling for Heated Cathode and Method of Manufacture, Ser. No. 418,591, filed Dec. 7, 1964, now US. Patent No. 3,400,294, granted Sept. 3, 1968, and assigned to the assignee of the present invention. Connection 6 joins the heater element 5 to terminal 7 formed of a suitable material such as titanium, while a thin cylindrical cathode support 8, desirably formed of tantalum or hafnium, completes a circuit between heater element 5 and terminal element 9.

The external portion of the envelope of the electron discharge device is formed of a plurality of ceramic insulating cylindrical members bonded to metal cylindrical terminal members. The ceramic insulating members are desirably forsterite because of its low dielectric constant, or a magnesia and magnesia alumina spinel, such as disclosed in US. Patent 3,113,846-Leschen assigned to the assignee of this present invention. The ceramic materials, moreover, have the same coeflicient of expansion as the titanium flange 10 which is bonded between ceramic cylinder 11 and anode 1. In accordance with my invention, the control electrode comprises a first relatively massive ring member 12, preferably formed of a high thermal conductivity metal, such as molybdenum, the member 12 also providing an external connection for the control electrode. Interposed between ceramic cylinder 11 and ring 12 are a pair of metal members 13 preferably formed of a suitablemetal, such as titanium and having a coefiicient of thermal expansion that matches the coeflicient of expansion of cylinder 11. Placed between members 13 is a shim 14 of molybdenum foil. One member 13 is sealed to ceramic cylinder 11 while the other member 13 is sealed to ring 12. The two members 13 are folded and joined together near their inner edges 15 as by brazing or welding to form a vacuum-type connection. This construction thus provides a vacuum-tight portion of the envelope at this point, while shim 14 permits and facilitates sliding movement between members 13 to accommodate for the unequal coefiicients of expansion of ceramic member 11 and ring 12 during operation of the electron discharge device. By this construction I am able to form ring 12 of a metal which has a thermal coeflicient of expansion which is only one half that of cylinder 11 and members 13.

Also in accordance with my invention, the cathode disk 2 and control electrode ring 12 are both mounted on accurately positioned reference surfaces comprising in FIGURE 1 the upper surface of ceramic member 16. It is not necessary that both members be mounted on the same single surface; instead they may be supported on a pair of accurately positioned surfaces of ceramic member 16. Sealed between the bottom surface of ceramic element 16 and a similar ceramic ring element 17 is resilient contact ring 18 formed of a suitable metal, such as titanium, having a coefficient of expansion which matches that of the ceramic members 16, 17. Contact ring 18, in turn, is brazed or otherwise joined to ring 12 at point 19. By this construction, flexibility and freedom of movement during expansion and contraction of the various members of the device is provided. Thus ring 12 which has a coefiicient of expansion greatly different from that of ceramic element 16 is free to move on the upper surface of that element while the flexibility provided by the curved portion of contact ring 18 prevents breaking the vacuum seal of the envelope at this point.

Control electrode 4 includes, in addition to ring elernent 12, ring element 20 of a high thermal conductivity metal such as molybdenum which is bonded to the upper surface of element 12 and has an inner diameter substantially the same as the inner diameter of element 12. Extending across ring element 20 are a plurality of relatively massive high thermal conductivity bars 21 which extend into the grooves 22 in the upper surface of cathode 2, being spaced from and electrically insulated from the walls of grooves 22. Extending across the ring 20 in the direction parallel with bars 21 are a plurality of very fine grid wires 23 and arranged transverse to and contacting grid wires 23 are thicker or larger grid wires 24. In accordance with my invention, members 12, 21 and 24 are made of a high thermal conductivity material, such as, for example, molybdenum, the wires 23 also being of a high thermal conductivity material, such as, for example, tungsten or molybdenum. As a result of this structure in the operation of the device at high power levels, the large high thermal conductivity members 21, 24 and 12 conduct heat readily from the very fine wires 23 so that wires 23 never reach a temperature at which they distort despite high power level operation of the device. Further, in accordance with my invention, I provide a plurality of tapped holes or apertures 25 in ring 12 by means of which this ring may be connected by bolts, not shown, to a conventional external circuit, also not shown, but which facilitates the removal of heat from the control grid structure and prevents overheating and distortion of the fine grid wires 23. The external circuit conventionally is a part of a cavity resonator which, when joined by bolts in the tapped holes 25, provides a large heat sink to which heat from the control electrode is rapidly transmitted through the high thermal conductivity materials. In this construction, the molybdenum wires 24 and the molybdenum bars 21 and rings 12 and 20 provide both high strength and low coeflicients of expansion While conducting heat rapidly from control grid wires 23 and 24 so that these wires are maintained taut under conditions of extreme heat, thereby maintaining a stable structure within the device. During such conditions the larger wires cool the fine wires by the thermal contact between the wires at their cross-over points. Member 18, which is formed of titanium and is used to form a portion of the seal of the device, has a loop portion 26 which flexes when the device is operated at high temperatures to prevent any undue strain on the seals of the device.

In a modification of my electron discharge device illustrated in FIGURES 3-6, instead of using tapped holes 25 of FIGURES 1 and 2 and external circuits for cooling the fine grid wires, ring element 12 includes a pair of apertures 32 into which are connected tubes or conduits 33, 34. Also, instead of using the solid ring element 20, I employ a ring element 27, which has undercut portions 28 connecting with tubes 33, 34. In addition, the solid bars 21 of the control electrode of FIGURE 2 are replaced by bars 29 which have central apertures 30 connected with undercut portions 28. In construction, ring member 27 may be formed of a plurality of laminations, as indicated in the drawings. By this construction a cooling fluid may be introduced through tube 33 to the undercut portions 28 of ring member 27 to flow through interconnected passageways 30 in :bars 29 and exit through conduit 34. As determined by the power level operation of the device and the highest temperature to which the control electrode is subjected, the cooling fluid may be either air, an inert gas, or a liquid, such as water. The particular cooling fluid employed, as well as its pressure, will vary with the operating conditions of the device.

In the operation of a device constructed in accordance with the configurations shown in FIGURES 1 and 2, I have found that a small size electron discharge device, having electrodes approximately one inch in diameter, is capable of operating at a power level of 2500 watts as contrasted with a maximum operating power level of 500 watts without cooling measures elfected through my invention. The construction, furthermore, permits the use of a rigid high thermal conductivity control grid structure which includes very fine grid wires. A resultant and important advantage is that I am able to obtain a stable, rigid structure at higher power outputs. Furthermore, the structure is one which permits the use of external circuits to cool the internal electrodes of the device. A particular advantage of the use of the single reference surface afforded by the upper surface of insulator 16 as a support for both the control electrode and the cathode is that, despite high operating conditions resulting in changes in the dimensions of the control electrode and the cathode members, distortion of the electrodes is eliminated.

In such operation, expansion of the cathode support cylinder permits cathode disk 2 to rise slightly above its mounting surface on insulator 16 and return without distorting the electrode or disturbing the no-load inter-electrode spacing. Employment of the overlying members 13 with shim 14 between them to permit movement between insulator 11 and control electrode ring 12, as well as the use of the sealing member 18 with its loop 26, assures that no seals of the device are disrupted despite the expansion and contraction of the various members of the device during operation at high power levels.

While I have shown and described several embodiments of my invention, it will be apparent to those skilled in the art, that many changes and modifications may be made without departing from my invention in its broader aspects. I, therefore, intend the appended claims to cover all such changes and modifications as fall within the true spirit and scope of my invention.

What I claim as new and desire to secure by Letters Patent of the United States is:

-1. In an electron discharge device having a planar anode and an opposed circular cathode having a plurality of spaced parallel grooves therein,

a control electrode comprising a relatively massive high thermal conductivity metal ring encircling said cathode, a plurality of high thermal conductivity bars supported by said ring and extending into the grooves in said cathode, and a plurality of high thermal conductivity conductors arranged transversely and joined to said bars, and

an annular insulator located below adjacent portions of said cathode and metal ring and having accurately positioned reference surfaces for supporting both said cathode and said metal ring for movement with respect to such reference surfaces.

2. In the device of claim 1, said high thermal conductivity metal ring being formed of molybdenum.

3. In the device of claim 2, said insulator having a bottom surface, and a flexible metallic contact member hermetically sealed to said bottom surface and said ring, said contact member and said metal ring forming a portion of an envelope of the device.

4. In the device of claim 2, means for thermally connecting said ring to an external cooling circuit whereby heat is conducted from said conductors and bars through said ring to such external circuit.

5. In the device of claim 2, said ring and said bars having interconnected internal passageways, and conduit means connected to said ring to pass a cooling fluid through said passageways.

6. An electron discharge device comprising a planar anode having a flange,

a circular cathode having a plurality of spaced parallel groves in a rst surface thereof opposed to said anode, a planar second surface, and means for heating said second surface,

an annular insulator having a planar reference surface positioned to engage the outer portion of said planar second surface,

a control electrode structure comprising a thick first high thermal conductivity annular metal member encircling said cathode and having a lower surface whose inner portion is supported on said planar reference surface,

a tubular ceramic cylinder sealed between said anode flange and the upper surface of said first annular metal member,

a second high thermal conductivity annular metal member joined to the upper surface of said first annular metal member within said cylinder,

a plurality of high thermal conductivity bars supported by said second annular metal member and extending into the grooves in said cathode, and

a plurality of fine grid wires extending across said second annular metal member and positioned between said cathode and said anode, said first and second annular metal members forming an externally accessible high thermal conductivity path for cooling said fine grid wires.

7. The device of claim 6 in which a plurality of thicker grid wires of a high thermal conductivity material are arranged transversely to said fine wires and thermally connected to said fine wires and said second annular metal member.

8. The device of claim 6 in which said first and second annular metal members and said bars have interconnected internal passageways, and conduit means connected to said first annular member for passing a cooling fluid through said passageways.

9. The device of claim 6 in which said tubular ceramic cylinder and first annular metal member have different coefiicients of expansion and the seal between said tubular ceramic cylinder and the upper surface of said first annular metal member comprises a pair of juxtaposed annular rings positioned between said cylinder and said member, one of said rings being sealed to said cylinder, the other ring being sealed to said first annular metal member, said rings being bonded together adjacent their inner edges, and a metal shim positioned between said rings whereby said cylinder and first annular metal member may expand and contract during operation without rupturing the seal therebetween.

10. In an electron discharge device having a planar anode and an opposed circular cathode,

a control electrode comprising a metal ring encircling said cathode and a plurality of wires connected across said ring and positioned between said anode and cathode, and

an annular insulator having accurately positioned reference surfaces for supporting said cathode and said metal ring,-

said insulator, ring and cathode having different coeflicients of thermal expansion and said ring and cathode as they expand and contract during operation of the device being free to move on said reference surfaces.

References Cited UNITED STATES PATENTS 10/1959 Millis 313261 8/1967 Beggs 313348 US. Cl. X.R. 313-338 

